The present invention relates to a net type heating device for heating a broad area such as a roof or a farm to keeping the broad area warm.
Conventionally, an electric heating device for heating a broad area or keeping a broad area warm uses a single electric wire arranged in a spiral or zigzag pattern. However, when a disconnection occurs at a part of the single electric wire of a conventional electric heating device, heating or warming is interrupted. Also, a conventional electric heating device of this type cannot be used for heating a broad area or keeping a broad area warm by low voltage and high current, because the current flow is limited by the single wire.
In order to solve the above problem, the present inventor provided a heating sheet in which a plurality of electric heating wires, which are substantially parallel to each other, are woven into one sheet, and the electric wires are connected to common power soruce terminals which are provided at both sides or the middle and sides of the sheets.
In the above heating sheet, however, the heat produced by the electric heating wires is collected in the heating sheet. Therefore, the heating sheet cannot uniformly heat the area between the electric heating wires. Also, the above heating sheet requires a large number of sheets for heating a broad area or keeping a broad area warm. Furthermore, the above heating sheet is not expandable and contractable.
A technique for obtaining a heating sheet in which nonconductive fibers and fine metal wires are woven in turn is known. The woven heating sheet does not possess suitable expansion and contraction ability and is not bendable for directly weaving the metal wires into the nonconductive fiber, resulting in inconvenience.